I have been chasing an electrical problem in my Daimler for several days and believe it to be the in the ignition switch. The switch is a Lucas PLC 6 combined ignition and lighting switch. The problem is that nothing electrical is working except the horn. Whilst I can’t follow it on the wiring diagram this is the way I think the ignition switch is connected. Power comes into the switch via the ammeter to terminal A. There is no connection to any other terminal in the switch when the key and light switch are off. When the key is turned on there should be a connection between terminal A and terminal IGN which feeds the coil. When the light switch is turned to the first position there should be a connection between terminal A and S (sidelights and tail lights). In the second position there should be a connection between A and S and H (headlights). Am I correct in these assumptions?

That sounds correct though I should say I don't have a wring diagram for a DB18 and can't confirm your letters.

The feed through the switch should be provided by 12 volts at terminal "A" and as you say when the key is turned the 12 volts should appear at terminal "ign" and from there to the coil.

I have a volt meter but when checking these circuits I prefer a side light bulb in an old holder with test wires attached, it is a lot easier than trying to hold a meter and prods in place when testing, but that is my personal preference.